When forming a minute resist pattern on a resist film on a semiconductor wafer by lithography, the focus position at the time of pattern exposure greatly influences the dimensional accuracy of the resist pattern. Thus, at the start of construction of a new process, wafers printed with patterns by changing focus values of exposing equipment are produced per unit exposure, and the dimension of the formed resist patterns is measured, thus setting the best focus value for obtaining a predetermined pattern dimension.
However, because of various process variations, it is unlikely that the best focus value that has been set can be used forever to obtain the resist pattern with a predetermined dimension. Therefore, in a conventional technology, a focus deviation has been corrected according to the variation in an optimal position of focusing caused by the process variation. For doing this, it is necessary to determine the focus deviation amount deviating from the optimal position. JP 2003-59813 A describes a method for determining such a focus deviation amount. This method will be explained with reference to FIGS. 19A, 19B, 20A, 20B and 21.
FIG. 19A shows a cross-section of a line pattern 1 formed of a resist remaining in a linear form. FIG. 19B shows a cross-section of a space pattern 2 having a space 3 that remains after removing a resist partially in a linear form. These resist patterns are used for observing how the shape of the formed patterns varies when shifting a focus at the time of pattern exposure using a mask in a positive direction and a negative direction from a best focus value. Here, the positive side of the focus value refers to a state in which a focus is achieved on a side below the best focal point, and the negative side of the focus value refers to a state in which a focus is achieved on a side above the best focal point.
The variation in the shape of the line pattern 1 and the space pattern 2 described above with respect to the focus variation becomes notable in the inclination amount of end faces of the pattern (in the following, referred to as an edge inclination amount). Now, EW1 denotes the edge inclination amount of the line pattern 1 shown in FIG. 19A, and EW2 denotes that of the space pattern 2 shown in FIG. 19B. With their horizontal axes indicating the focus value and their vertical axes indicating the edge inclination amounts EW1 and EW2, FIGS. 20A and 20B show lines indicating the focus dependence of the edge inclination amount, namely, a variation in the edge inclination amount with respect to the variation in the focus value. Each line corresponds to a measurement result obtained by each of exposures with different exposure amounts. The focus value 0 corresponds to the best focus.
As shown in FIG. 20A, the edge inclination amount EW1 of the line pattern varies notably when the focus is shifted from the best focal point in the negative direction. On the other hand, as shown in FIG. 20B, the edge inclination amount EW2 of the space pattern hardly varies when the focus is shifted from the best focal point in the negative direction but varies notably when the focus is shifted to the positive side. From these edge inclination amounts EW1 and EW2, a model showing a focus dependence of the edge inclination amount can be formulated as shown in FIG. 21. In other words, by obtaining the difference between EW1, which varies on the negative side, and EW2, which varies on the positive side, i.e., EW1—EW2, it is possible to obtain a curve showing the characteristics of the variation in the edge inclination amount with respect to the variation in the focus value. Using this curve, a focus deviation amount can be determined from the correspondence with the edge inclination amount.
However, as becomes clear from FIG. 21, there is a region in which the value of the vertical axis (EW1−EW2) remains flat near the best focus value (=0 μm). Thus, with this method, the focus deviation amount cannot be determined accurately near the best focus value.
The formation of this flat region is attributable to two factors. First, the resolutions of the line pattern and the space pattern are different in the current resist. Usually, the focus is achieved on the line pattern, but this causes the space pattern to be out of focus. In other words, the best focus values of the line pattern and the space pattern do not match. For example, the best focus value of a 0.18 μm line pattern does not match with that of a 0.18 μm space pattern.
Second, there is a dimensional error in manufacturing masks. In a factory where plural types of products are produced, masks are manufactured for each type. When manufacturing plural kinds of masks, dimensional errors occur inevitably. Such dimensional errors of the masks result in a problem that the best focus values of the line pattern and the space pattern do not match.